Comment 2 for bug 675469

Quoting from:
man time

"...
ACCURACY
       The elapsed time is not collected atomically with the execution of the
       program; as a result, in bizarre circumstances (if the time command
       gets stopped or swapped out in between when the program being timed
       exits and when time calculates how long it took to run), it could be
       much larger than the actual execution time.

       When the running time of a command is very nearly zero, some values
       (e.g., the percentage of CPU used) may be reported as either zero
       (which is wrong) or a question mark.

       Most information shown by time is derived from the wait3(2) system
       call. The numbers are only as good as those returned by wait3(2). On
       systems that do not have a wait3(2) call that returns status
       information, the times(2) system call is used instead. However, it
       provides much less information than wait3(2), so on those systems time
       reports the majority of the resources as zero.

       The `%I' and `%O' values are allegedly only `real' input and output and
       do not include those supplied by caching devices. The meaning of
       `real' I/O reported by `%I' and `%O' may be muddled for workstations,
       especially diskless ones.
..."